Archerfish are the Robin Hoods of the aquatic world – they rarely miss their target. But unlike Robin Hood, they are less keen to share their loot.

Archerfish catch food by firing jets of water at prey. They share waters with more numerous fish called halfbeaks, which are after the same prey – typically grubs, crickets or beetles – and will snatch insects downed by archerfish if given a chance.

To survive, archerfish have had to evolve new, sharper hunting skills such as “predictive start”, whereby they launch themselves towards where the prey will hit the water while it is still falling. But although this helps them to beat halfbeaks during the day, archerfish have had to give up hunting at night, when halfbeaks’ skills come to the fore.

The discovery of this “arms race” between archerfish and halfbeaks was made by analysing hundreds of high-speed video recordings of archerfish in action, both in their mangrove habitat in Thailand and in the lab.

“At every spot, archerfish were found together with other surface-feeding fish, and typically these rivals not only outnumbered the archerfish, but also fed on the same prey that archerfish would down with their shots,” says Stefan Schuster of the University of Bayreuth in Germany, whose team conducted the research. “So it has become challenging for the archerfish to secure their prey.”

Specialist equipment
As part of the arms race, both species have developed specialisations to give them an edge.
Halfbeaks have evolved superior machinery for locating and snaffling prey in the dark, which seems to have driven archerfish to abandon hunting at night.

Lab analyses of skin from both species showed that compared with archerfish, halfbeaks have around five times more neuromasts on their backs – these are sensory cells that are vital for detecting waves triggered by prey landing on the water surface above.

Archerfish are keen-eyed killers (Image: Kim Taylor/naturepl.com)

In lab tests, archerfish did hunt at night in the absence of halfbeaks. They spotted potential prey in near total darkness, hitting and knocking it into the water almost every time.

But they struggled to locate the prey after they had knocked it into the water, taking much longer than halfbeaks do.

To make up for forsaking midnight snacks, archerfish have evolved superior visual skills that enable them to anticipate where “shot” prey will fall. During the daytime, this enables them to outpace their rivals to the spot where the unlucky insect falls.

“The halfbeaks show visual skills too, but they’re far less efficient than the archerfish,” says Schuster. “Archerfish win when they can use vision, but halfbeaks win in the dark.”

Food fight

Schuster and his colleagues discovered this in experiments where they deliberately tipped food into the water and then videoed the scramble.

“Archerfish were extremely efficient at making the catch in almost all cases by quickly noticing the initial speed and direction of the falling prey and heading right to the point where it would drop,” says Schuster.

On average, it took the archerfish just 90 milliseconds to get there, compared with 253 milliseconds for the halfbeaks.

“Our study shows that competition with other species can play an important role, forcing animals into new niches only available through cognitive investments,” says Schuster.

“Between-species competition appears to be driving the evolution and refinement of highly sophisticated behaviours in ways few would have predicted,” says Aaron Corcoran of Wake Forest University in Winston-Salem, North Carolina. “It’s remarkable that a fish species, the halfbeak, has apparently evolved specialised mechanical and visual sensing abilities to steal food from another fish species.”

Seabirds called shearwaters manage to navigate across long stretches of open water to islands where the birds breed. It’s not been clear how the birds do this, but there have been some clues. When scientists magnetically disturbed Cory’s shearwaters, the birds still managed to find their way. But when deprived of their sense of smell, the shearwaters had trouble homing in on their final destination.

Smell wouldn’t seem to be all that useful out over the ocean, especially with winds and other atmospheric disturbances playing havoc on any scents wafting through the air. But now researchers say they have more evidence that shearwaters are using olfactory cues to navigate. Andrew Reynolds of Rothamsted Research in Harpenden, England, and colleagues make their case June 30 in theProceedings of the Royal Society B.

Messing with Cory’s shearwaters or other seabirds, like researchers did in earlier studies, wasn’t a good option, the researchers say, because there are conservation concerns when it comes to these species. Instead, they attached tiny GPS loggers to 210 shearwaters belonging to three species: Cory’s shearwaters, Scopoli’s shearwaters and Cape Verde shearwaters.

But how would the birds’ path reveal how they were navigating? If they were using olfactory cues, the team reasoned, the birds wouldn’t take a straight path to their target. Instead, they would fly straight for a time, guided in that direction by a particular smell. When they lost that scent, their direction would change, until they picked up another scent that could guide them. And only when a bird got close would it use landmarks, other birds and the odor of the breeding colony as guides. If the birds were using some other method of navigation — or randomly searching for where to go — their paths would look much different.

When the researchers analyzed the paths of the shearwaters, 69 percent of the birds moved in a way that matched what was expected if they were using olfactory cues. Nearly all of the journeys that lasted four or more days took this kind of path, but less than half of short flights that lasted less than two days had this kind of flight path.

“All these animals share the same basic pattern,” the researchers write, “strongly suggesting the presence of an underlying common mechanism of orientation which we have identified as olfactory-cued navigation.”